Welding robots are widely used in a mass-production line for the spot-welding of, for example, automobile bodies, as workpieces The appearance and performance of the robot unit of such a welding robot is described in, for example, a catalog published by Fanuc Ltd. (Head office: Yamanashi-ken Japan) "FANUC ROBOT S Series", p. 14, Oct, 1989. A spot welding robot incorporated into such a mass-production line produces a spot-welding energy by transforming an electric primary voltage into a secondary voltage through an electric transformer, and applying the secondary voltage across the electrodes of the spot welding gun. Since the transformer is very heavy, in general practice the electric transformer is installed outside the framework of the welding robot, i.e., a robot unit, and only the spot welding gun is mounted on the robot wrist in such a manner that the secondary voltage of the transformer is supplied to the spot welding gun through a secondary welding cable extended between the transformer and the robot unit.
Nevertheless, since the secondary welding cable is extended between the spot welding gun mounted on the robot wrist and the transformer installed apart from the robot unit, and because the welding cable used must be a heavy cable, due to a large current capacity thereof, the movement of the movable components of the robot unit such as the robot arm or the robot wrist, is obstructed by the welding cable if the robot body portion or the robot arm of the robot unit becomes entwined with the welding cable.
An improvement has been made, in an attempt to solve the problems attributable to the entwinement of the movable robot components with the welding cable, in which the transformer is disposed at a particular position on the robot unit, such as near the extremity of the robot wrist. Nevertheless, there is a need for an improvement of the production efficiency of the welding process in the mass-production line, through the curtailment of the processing time to the least possible extent by mounting two spot welding guns on a single welding robot to form a plurality of weld spots in a single spot-welding cycle.
When the two spot welding guns, each provided with an electric transformer are mounted on the robot wrist or a part in the vicinity of the robot wrist, however, the gravitational load on the robot wrist and other movable robot components is greatly increased, and accordingly, the robot unit becomes unable to fully exhibit its functions and the entire robot performance is restricted.